Doubt about pre-operative carbohydrates

We commend Fawcett and Thomas for their review of preoperative fasting recommendations [1] and applaud their candid acknowledgement of the mounting evidence of lack of clinical benefit for oral preoperative carbohydrate loading (preCHO), considered an essential element of the Enhanced Recovery After Surgery (ERAS) Program [2].

We wish to mount further direct challenges to the concept of preCHO, on several grounds. The subjective benefits of preCHO, namely the reduction in anxiety, distress, thirst and hunger [1-3], are relative to the dietary habit of the subject during the preceding weeks, as much as they are to the immediate duration of restriction of food and water. A predominantly carbohydrate-based ‘standard’ diet (such as has been advised by national advisory bodies for several decades) accentuates these symptoms, while the widespread adoption of reduced fasting times for both food and water (6 and 2 hours, respectively), reduces the impact of this acute deprivation. Indeed, preCHO has been shown to be of benefit only when compared to fasting without water, but negligible when compared to water [1]. The analogy drawn between surgical stress and exercise, with respect to lactate production and carbohydrate loading, is both false and outdated. In the context of exercise, lactic acid is produced when glycolysis outstrips mitochondrial capacity for aerobic metabolism, whereas intra-operative lactic acidosis is likely to represent hypoperfusion. This does not imply a primary substrate lack and therefore is not improved by preCHO.

The objective metabolic benefit of preCHO is purported to be a reduction of perioperative Insulin Resistance (IR), as evidenced by an improvement of Insulin Sensitivity assessed by Hyper-Insulinaemic Euglycaemic Clamp test (HIEG) [3]. We contend that this appears to be based on a divergence from the metabolic standards of that test. When the HIEG was first described in 1979 [4], the pre-conditions required that all subjects not exercise for 48 hours and all consume “at least 200g of carbohydrate per day for three days prior to study”, implying that some standardisation of carbohydrate consumption is necessary. However, in all of the preCHO-studies’ application of the HIEG [3], precisely the opposite occurs: the preCHO group is administered 150g maltodextrin within 12 hours of the preoperative test, while the fasted control group is not. It is our contention that this preCHO augments the disposal of glucose during subsequent HIEG, through mechanisms that are not yet fully understood. By way of analogy, this is akin to comparing sprint time-trial performance between those that are sprinting at the start, versus a stationary-start control.

Since preCHO has no significant demonstrable clinical benefit compared to placebo, we propose that it be relegated to be an optional, rather than obligatory, component of ERAS. Further, in light of current understanding of the incidence and consequences of perioperative hyperglycaemia [5] and the growing recognition of the incidence and implications of covert primary hyperinsulinaemia [6] in the population, we would caution against preCHO in the majority of surgical patients.

Doubt about pre-operative carbohydrates - a reply

We are grateful for the kind interest shown in our article by Scott et al [1]. We did not intend to suggest that the pathophysiology of major surgery and high performance athletes were similar, except that both groups have carbohydrate loading in common, and that lactic acid accumulation is disadvantageous to both.

Concerning insulin resistance (IR), the gold standard for measuring IR remains the hyperinsulinaemic–euglycaemic clamp test, which has been extensively described for perioperative patients for over 20 years [2]. This is a dynamic test in which a high-dose insulin infusion is administered, thereby suppressing hepatic glucose production, with the concomitant required glucose rate to maintain euglycaemia defining metabolizable glucose. Subjects with normal insulin sensitivity requiring relatively higher rates of glucose, whereas subjects with IR require much lower rates of glucose to maintain euglycaemia. This test is undoubtedly superior to the other methods eg homeostatic model assessment in which in only fasting plasma glucose and insulin concentrations are measured and IR is inferred from the product of these two measurements, with increasing values interpreted as increased IR.

We agree with the comments about perioperative hyperglycaemia, and quoted the article by Kwon to support this [3], yet the clinical conundrum is whether or not to avoid carbohydrate loading, or to give it and then treat any resultant hyperglycaemia. As stated, a reasonable approach is to administer carbohydrates and then treat hyperglycaemia (as one would hyperglycaemia from any cause).

Finally, a major, uncertainty remains the precise impact of each of the twenty or so individual elements of ERAS. Clearly, they are not all of identical weighting, with some of elements having a much greater impact than others. Recently, Henrik Kehlet, who is credited with having pioneered and developed the concept of ERAS, recently wrote an editorial stating that the five major areas to focus on are pre-operative patient information, thoracic epidural (for open abdominal/pelvic surgery), avoidance of fluid overload and hypovolaemia, no nasogastric tube and early oral feeding and mobilisation, with no mention of carbohydrate loading [4.

What is the place, therefore, of carbohydrate loading? We would not support Scott et al.s’ assertion that carbohydrate loading has “no significant demonstrable clinical benefit compared to placebo”, as two large meta-analyses support its use [5,6], even though a later meta-analysis disputed these [7]. One explanation may be that as overall care improves within ERAS programmes, the benefit conveyed by of any one ERAS element may be attenuated, with its signal of conferred benefit becoming lost. An example of this is Goal Directed Fluid Therapy (GDFT) in major abdominal surgery, which, when compared to conventional fluid therapy, caused a significant reduction in postoperative complication rates and length of stay prior to the introduction of ERAS pathways. However, after ERAS programmes were established, these benefits were no longer apparent [8].

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